1. Field of the Invention
The present invention relates to a device for detecting a position of an object and, more particularly, to a device for detecting a positional deviation or displacement between a photomask or reticle and a wafer in an alignment and exposure apparatus for manufacturing semiconductor circuits, or to a device for detecting a position of a semiconductor device relative to a machining apparatus or an inspecting apparatus.
2. Description of the Prior Art
In the field of manufacturing semiconductor circuits, the positions of an alignment mark formed on a mask, which will hereinafter be called a mask alignment mark, and an alignment mark formed on a wafer, which will hereinafter be called a wafer alignment mark, must be detected with high accuracy in order to precisely align the mask and wafer, prior to exposing the wafer to the semiconductor circuit pattern of the mask or reticle.
It is known in the prior art that, in order to bring the mask and the wafer into alignment with each other, the mask alignment mark and wafer alignment mark may be scanned with a laser beam to detect their positions. For example, U.S. Pat. No. 4,199,219 proposes that a light beam scans a mask and a wafer which are already roughly aligned, to scatter the light reflected by the alignment marks. The reflected light is sensed by a photodetector, so that the positions of the alignment marks are converted to time intervals of the photodetector outputs. This system requires that the laser beam scans the mask surface and the wafer surface, and more particularly the mask alignment mark at a constant speed and the wafer alignment mark. However, it is difficult to scan them at a constant speed because each of the plural mirror surfaces of a polygonal mirror has to be positioned extremely accurately, and also because there is an unavoidable non-uniformity in the polygonal mirror rotation. This results in a measurement error in the positions of the mask alignment mark and wafer alignment mark.
U.S. Pat. No. 4,262,208, which has been assigned to the assignee of the present application, proposes a device which is capable of detecting a position without using scanning means. Even in such a system, however, when the intensity of the light reflected by the wafer, particularly the defracted light from the marks, is not high enough, the level of the light signal is so low that the position thereof is not detected.
Further, a recent trend in semiconductor manufacturing causes the reflected light signal level to be too low to detect the mark position (1) because a multi-layer photoresist is used resulting in a lower intensity of reflected light and/or (2) because the pattern is finer, resulting in a lower intensity of light reflected by a ridge or score of an alignment mark which is too low or shallow, respectively.